May and Spring Climate Summaries for Central and Southeast Illinois

Typical May weather occurred across Central and Southeast Illinois during 2011 with occasional bouts of thunderstorms and severe weather.The heaviest rainfall across the region fell northwest of the Illinois River where precipitation for the month was generally 1.5 to 3 inches above normal.Peoria recorded 2.79 inches of rain on the 25th alone.Southeast Illinois had several rounds of heavy rainfall resulting in flooding along the Wabash River during the first week of the month.

Temperatures were also pretty typical for May with averages in the 60s.Cooler periods occurred near the beginning of the month and again in the middle, with temperatures well above normal May 9-12.

As is typical for May, periods of severe weather also impacted the region.Nine tornadoes occurred across Central and Southeast Illinois during May.Some of the more significant events are highlighted below:

May 1-3 Review

Rain fell across east-central and southeast Illinois from Sunday, May 1st through Tuesday, May 3rd as a stationary frontal boundary plagued the area. While most of central Illinois received less than one quarter of an inch during this period, locations along and south of I-70 picked up in excess of 1 inch. In fact, areas along and southeast of a Robinson to Flora line observed over 2 inches.This rain fell on already saturated ground from heavy rain during the latter half of April.

Due to excessive amounts of rain during April and early May, many rivers and streams went into flood across southeast Illinois. In particular, serious flooding occurred along the Wabash and Embarras Rivers.

May 13 Review

A deep upper-level low tracking across southern Missouri produced several bands of showers and thunderstorms across central Illinois on Friday, May 13th. Thanks to weak steering currents aloft, the storms were very slow-moving and thus produced locally heavy rainfall in a few spots. Most notably, portions of Marshall, Woodford, and Tazewell counties picked up 2 to 4 inches of rain during the evening of the 13th into the early morning hours of the 14th. In addition to the rain, quarter-sized hail was reported in Hopedale and Tremont in Tazewell County.

May 25 Review

Two rounds of strong to severe thunderstorms impacted Central Illinois on May 25th. The first occurred during the pre-dawn hours, while the second developed during the late afternoon and evening. Widespread wind damage and 8 tornadoes occurred.

The early morning storms formed well north of an advancing warm front and were fed by strong low-level winds extending from southwest Missouri into central Illinois. Despite relatively cool and stable conditions at the surface, the transport of warm/moist air aloft helped feed the thunderstorms as they tracked northeastward through the area. Thanks to the strong winds a couple thousand feet aloft, many of the cells were able to produce damaging wind gusts of 50 to 60 mph all the way to ground level. Periodic wind damage was observed in a corridor from just west of Jacksonville northeastward through Delavan in Tazewell County. Other wind damage was concentrated across parts of Douglas, Champaign, and Vermilion counties. A more intense thunderstorm cell produced two tornadoes across eastern Macon County, damaging a number of outbuildings and grain bins east of Decatur.

After a brief lull around midday, additional storms began to fire along an outflow boundary left behind by the morning activity. This boundary generally stretched along the I-70 corridor from near Paris to just south of Taylorville. Many of these storms produced small hail and gusty winds, with the strongest cell dropping golfball-sized hail in Paris.

By late afternoon, a deep upper-level low began approaching from the west. Surface warming in conjunction with cold air aloft associated with this feature led to an increasingly unstable airmass across much of the Midwest. Supercell thunderstorms rapidly developed in this favorable airmass across Missouri and west-central Illinois after 4 PM, then began slowly shifting eastward toward evening. Numerous reports of wind damage and hail were received, along with intense rainfall rates leading to localized flash flooding. Once the storms pushed into east-central Illinois by early evening, they had congealed into a linear structure with many weak circulations embedded along the leading edge. Widespread wind damage occurred with this line of storms, with several spin-up tornadoes developing.

Several rounds of storms impacted the region over the Memorial Day weekend with wind damage, hail, and very heavy rainfall.On the 28th, A corridor of hail as large as golf balls was reported from near Jacksonville eastward into the northwest side of Springfield.A report of golfball-sized hail was also reported in Charleston from a supercell thunderstorm that crossed the state from just north of St. Louis eastward to Paris.

On the 29th a midday thunderstorm complex brought flash flooding to the Rio area of Knox County and isolated hail and wind damage reports north of U.S. Highway 136 in Central Illinois.

As for precipitation, rainfall amounts for the season were typically from near normal to slightly above normal.However, there were definitely some significant rainfall events that are not apparent in the seasonal normal.Of special note are the flooding rains across southeast Illinois during late April and early May, and the heavy rainfall in the Peoria area where 7.33 inches fell during April making the month the fifth wettest April on record.In addition, 2.79 inches of rain fell at Peoria on May 25th.

Eighteen tornadoes were reported across Central and Southeast Illinois during the period with the strongest (Rated EF2) occurring in Crawford County during the evening of May 25th.